Implantable pressure monitor

ABSTRACT

An implantable pressure monitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/452,920, filed Jun. 15, 2006, which claims the benefit of U.S.provisional application Ser. No. 60/738,980 filed Nov. 23, 2005, and ofU.S. provisional application Ser. No. 60/773,344, filed Feb. 15, 2006,the disclosures of which are hereby incorporated herein by reference.

FIELD

The invention is directed generally to a method and apparatus and forsensing a characteristic of a patient, such as blood pressure and/ortemperature, and more particularly to methods and devices particularlyadapted for telemetric measurement of blood pressure via a deviceimplanted within the cardiovascular system during surgery andparticularly within the heart.

BACKGROUND

The National Institute of Health (NIH) has concluded that heart failureconstitutes “a new epidemic” in the USA. Heart failure, a chronic,progressive and incurable disease, affects over 20 million peopleworldwide. In the US alone, some 5 million people have been diagnosedwith heart failure. Heart failure is estimated to cost the US economytoday more than $40 billion annually.

Intracardiac pressure management is an important aspect of heart failuretreatment. For example, a rise of the intracardiac pressure, such as inthe left atrium is an important early indication of disease progressionand the first opportunity for therapeutic intervention. Current bloodpressure-measuring methods only can be applied in the coronary care unit(CCU) or the intensive care unit (ICU) and provide no more than anoccasional snapshot of intracardiac pressure when the patient is alreadyin a very critical situation. The limitations on current intracardiacpressure measurement methods are a serious impediment to early andoptimal treatment. Current treatment methods require hospitalization andmay be extremely costly (on average, over $16,000 per patientadmittance). The ability to monitor patients and intervene outside ofthe hospital setting would greatly reduce the number of hospitalizationsand extend the lives of those affected by the diagnosis.

Various sensors and devices have been used or proposed for themeasurement and analysis of the blood pressure and/or temperature of apatient with mixed success. The currently contemplated sensors havecertain disadvantages. For example, the telemetric sensor described inU.S. Pat. No. 6,855,115 can be implanted in the heart by a catheter andis not designed for surgical implantation. Moreover, the sensor, whichis rolled up during the implantation procedure, must be made of aflexible material of a specific configuration so that any change of theblood pressure inside the heart effectuates a change in the distance ofthe sensor height, i.e., the distance between the two capacitor platesused in the sensor. This flexible sensor is folded for delivery via acatheter and then unfolded at the place of implantation. However, adisadvantage of such a configuration is its required flexibility asconstant and precise acquisition of measurement data may not be possiblewhen the sensor is placed on or close to the cardiac muscle, andtherefore is exposed to the cardiac motions, which may influence correctpressure readings. In addition, the flexible material of a sensor madein accordance with U.S. Pat. No. 6,855,115 may deform due to exposure toconstantly streaming liquids, especially a turbulent blood stream likelyencountered inside the heart. As a consequence, the capacitance of thecapacitor may be changed and measurement values may deteriorate and/ordeviate from the true value. Another disadvantage of this type of sensoris due to its use of a pressure-dependent LC-oscillator. The resonantfrequency of this oscillator can be analyzed telemetrically. Inprinciple, this kind of device can be applied to measure the pressurethat affects the measurement capacitor. Thus, any damage to the materialcan affect the pressure measurements obtained. Further, as the sensor isinfluenced by the surrounding media of the sensor, a corruption ofmeasurement values may occur. In addition, there is no circuitry in thistype of sensor to digitize the pressure measurement values acquired.Using analog signals may result in external interference during theacquisition and transmission of data, which causes inaccuracies inreadings.

Another exemplary implantable device, described in U.S. Pat. No.6,409,674, uses a catheter filled with a pressure transmitting fluid orgel-like material. The catheter transmits pressure to a pressuretransducer within a housing. The sensed pressure is then telemetricallytransmitted to an external reader. However, such a device requires ahousing for the electronic signal processing circuitry, which results ina larger and heavier sensor structure that can cause strain on the heartwhen implanted into a heart wall. Moreover, the catheter and housingconfiguration creates a more complicated, mechanical structure that maybe at increased risk for mechanical failure, and therefore is notsuitable for long term implantation.

Another device, described in U.S. Pat. No. 6,970,742, has a pressuresensor placed within the heart. A signal from the pressure sensor istransmitted to a housing outside the heart which contains the electronicprocessing circuits. The signal is processed by the electronicprocessing circuits, such as converting the signals from analog todigital, and then telemetrically transmitted to an external reader.However, housing the electronic processing circuitry requires additionalcomponents and a relatively larger implanted device. Moreover, becausedigitization of the signal does not occur until outside of the heart,there is a risk of interference in the wire connecting the sensor andthe electronic processing circuitry, as analog interference may resultfrom external sources.

Small pressure sensor chips including the electronic processing circuitshave been used in other applications. For example, integrated chipshaving pressure sensors have been used for pressure measurement inoptical and cranial applications. These sensors are compact and havefewer mechanical components. Examples of such pressure sensor chips aredescribed in EP 1 312 302 A2 and German patent application DE 10 2004005 220.7, in which the inventors of the present invention wereinvolved. However, these integrated chips are used in a relativelystable environment, with little movement in the fluids of the eye orbrain. Nor are these pressure sensors subject to the cyclical, dynamicmovements found in the heart. Such movement may harm connections, suchas connections between wires and the pressure sensing chip. Thus, theuse of such pressure sensor chips is not suited for the environment ofthe heart, where there is cyclical and dynamic movement, and where thereis continuous and turbulent fluid movement around the pressure sensor.

Conventional techniques to provide stability and support to such knownpressure sensing chips to enable their use as a cardiovascular pressuresensors would not likely succeed. Directly attaching a wire to apressure sensing chip may have a negative impact on the functionality ofthe chip. For example, when soldering is used for the connection, theheat may damage the chip. One known method of avoiding that problem isto adhere a substrate to the back of the pressure sensing chip, solderthe wire to a bond tack on that substrate, and then connect the wire tothe chip. However, such substrates have different coefficients ofthermal expansion than the chip. Thus, as the temperature changes, thesubstrate expands and contracts at a different rate then the pressuresensor chip, thereby causing stress and strain on the pressure sensingchip and increasing the risk of damage and/or inoperability.

Other known pressure sensors require a cable connection between thepressure sensor inside the heart and the external body monitoring deviceHowever, such a cable clearly requires an entry into the body. An entrymay be inconvenient and require the implantation of both the device andthe entry, as well as increase the risk of infection for the patient.

Thus, there is a need for intra-cardiac pressure sensors that are morereliable and accurate, and which cause less irritation when implanted inthe heart and are more compatible with the dynamic conditionsencountered in a moving heart. Also, a need exists for such a sensor tobe used at other locations within the cardiovascular system with littleor no modifications.

SUMMARY

The invention meets the above needs and avoids the disadvantages anddrawbacks of the prior art by providing a substantially rigid,chip-based telemetric sensor and system in which an extremely small andlightweight chip, including at least one pressure sensor and allnecessary electrical circuitry, may be implanted into the heart or otherportion of the cardiovascular system during surgery, to monitor bloodpressure and/or temperature.

In this manner, pressure signals may be digitized at or near the sensinglocation in the heart or other location in the cardiovascular system anddata may be telemetrically directed to the place of data acquisition toreduce or eliminate data transmission interference from externalsources.

In particular, the chip may be a substantially rigid structure thatprovides improved durability, long term stability, and long termaccuracy, and resistance to damage or a change in membranecharacteristics from the blood flow due to turbulences and the likewithin the bloodstream. For example, the chip may be an applicationspecific integrated chip (ASIC) containing all the necessary sensingelements and digital signal processing electronics. The ASIC preferablyis very small and lightweight to avoid undue stress on the heart and isorientated within the body in a position to minimize turbulent flow andreactionary forces. The ASIC may be used with an antenna in the form ofa coil created with very small dimensions. This minimal configuration ofASIC and coil may reduce and/or eliminate mechanical tensions effectingthe connection between ASIC and a coil.

The ASIC and the coil may be electrically and physically connected by aflexible coil. The ASIC, cable and coil may be encapsulated within aseamless biocompatible and flexible sheathing, such as silicone orsimilar material, to form an integrated sensor unit. The seamlesssheathing may maintain the integrity of the sensor by reducing oreliminating the exposure of the sensor to body fluids, such as blood. Itmay also be shaped and/or orientated to reduce turbulent flow.

A liquid or gel may be placed between the pressure sensing elements,such as capacitive membrane sensors of the sensor and the sheathing, toreduce or eliminate the effects of endothelialization on the surface ofthe sensor. The sheathing material itself may act as a pressuretransmitting material. The liquid or gel allows for integrating thepressure across the entire area of pressure sensing portion of thesensor to minimize the effects of localized plaque orendothelialization. Of course, heparin and other preventative coatingsknown in the art also may be used to prevent or reduceendothelialization.

To protect the ASIC and particularly the membrane sensor elements fromdamage due to handling, e.g., as a consequence of contact with asurgical instrument during implantation and/or during use, the sensordesign may have a unique geometry. For example, the ASIC may beconnected to a substantially rigid substrate in a spaced apartrelationship from the ASIC such that the substrate is opposite thepressure sensing elements of the sensor chip, with an aperture in thesubstrate providing access to the pressure elements to expose them tofluid pressure to be sensed. A silicone or other similar flexiblematerial may be disposed between the ASIC and the substrate. Moreover, apressure transmitting material may be placed within the gap between theASIC and the rigid substrate so that pressure from the blood can betransmitted to the pressure elements via the material.

The ASIC may incorporate a robust system to compensate for drift due tothe age and use of the sensor. For example, the ASIC may includeinactive pressure sensing elements that determine the change in themeasurement in the sensor due to age and usage, and may account for thischange when active pressure sensing elements determine the pressure.

The ASIC may be supported in a holder particularly adapted for anchoringthe ASIC in a wall of the heart or other location in thecardiovasculature. The holder may include a stop to position the ASICand limit its movement.

The ASIC is powered by induction from a wireless signal from an externalreader, thereby avoiding the need for an internal power source. Use of atransponder power supply at the external reader allows for asubstantially rigid sensor device with a longer life. The externalreader provides power to the substantially rigid sensor and receivespressure and temperature information from the substantially rigidsensor. The external reader stores and displays measurement andparameter data, calculates certain values. The external reader storesand displays measurement and parameter data, and may transmit the datato a computer or other device for further processing. The externalreader may have a separate antenna coil to facilitate prolonged periodson a patient's body. The external reader may store one or morecalibration curves for different sensors.

The telemetric pressure and/or temperature sensor of the invention maybe used for continuous or on demand sensing. A specific identificationnumber may be transmitted with each single measurement or measurementcycle. In this way, a continuous measurement value and sensoridentification, and therefore the measurement value and the identity ofthe patient, is provided. The identification number may allow a singleexternal reader to receive data from multiple sensors and systems and toassign them to the correct calibration curve for that sensor system andthe patient.

The invention may be implemented in a number of ways. According to oneaspect of the invention an intra-cardiac pressure measuring system formeasuring blood pressure inside the heart of a patient includes anantenna and an integrated chip. The integrated chip may include a firstsubstantially rigid substrate, at least one pressure sensor disposedwithin the substrate to generate signals indicative of a sensedpressure, and electronic signal processing components to process thesignals generated by the at least one pressure sensor. The electronicsignal processing components may be operatively connected to the antennaand the integrated chip may be powered by a signal received at theantenna. An implantable holder may support the integrated chip andinclude an anchor structure to mount the integrated chip within a wallof the heart during surgery such that the at least one pressure sensoris exposed to blood flow in the heart. The system may also include aremote receiver, wherein the integrated chip is operative to senddigital signals indicative of the pressure sensed in the hearttelemetrically via the antenna to the remote receiver.

The at least one pressure sensor may be capacitive-based pressuresensitive membranes housed within the substrate. The at least onepressure sensor may generate an analog signal in response to a sensedpressure and the electronic signal processing components may include atleast one analog to digital (A/D) converter to digitize within the heartthe analog signals from the at least one pressure sensor. The system mayfurther include a flexible wire connecting the antenna and theintegrated circuit, with the antenna being configured to be implantedwithin the patient underneath the skin to facilitate telemetric datatransmission to the receiver. The integrated chip may weigh less thanabout one gram, have a surface area on one side of less than or equal toabout 10 mm² and have a thickness of less than about 1 mm.

The antenna, the chip, wire and holder may be encapsulated in aseamless, one-piece biocompatible sheathing. A pressure transferringmedium may be interposed between the biocompatible sheathing and the atleast one pressure sensor. The biocompatible sheathing may act as thepressure transferring medium, and may be shaped to minimize turbulencein blood flow within the heart. The integrated chip may further includea unique digital identification, which is sent telemetrically to thereceiver. The receiver may obtain calibration information associatedwith the integrated chip based on the unique digital identification. Thereceiver may include a stored parameter and produce an alert based onthe signals indicative of the pressure sensed in the heart and of thestored parameter.

The system may further include a second substantially rigid substratelocated opposite the at least one pressure sensor in the first substrateand in a spaced apart configuration to protect the chip from mechanicaldamage. The second substrate may include an aperture permitting bloodflow within the heart to act on the at least one pressure one pressuresensor. A pressure transferring medium may be interposed between the atleast one pressure sensor and the second substantially rigid substrateto transfer blood pressure to the at least one pressure sensor. At leastone bond pad may be disposed between the first and second substantiallyrigid substrates and electrically connected to the integrated chip. Atleast one bond tack may be provided on the second substantially rigidsubstrate and be connected to the at least one bond pad such that theantenna is operatively connected to the integrated chip via the at leastone bond pad and the at least one bond tack to provide a strain reliefconnection. An antenna connector may connect the antenna to the secondsubstantially rigid substrate, wherein the antenna connector includes asignal portion electrically connecting the antenna to the integratedchip and a support portion connected to the second substantially rigidsubstrate. The antenna connector may be attached to the secondsubstantially rigid substrate such that there is slack in the signalportion when the support portion is taut. The signal portion may beconnected to the integrated chip via the at least one bond tack and thesupport portion may be connected to the second substantially rigidsubstrate via an opening in the second substantially rigid substrate.The at least one bond tack may include at least two bond tacks disposedon opposite sides of the aperture. The second substantially rigidsubstrate may include a protective barrier connected thereto, and abiocompatible sheathing may encapsulate at least the integrated chip andthe second substantially rigid substrate. The protective barrierprevents the first substantially rigid substrate from puncturing thebiocompatible sheathing. A flexible support material may be providedbetween the integrated chip and the second substantially rigidsubstrate.

The holder and the first substantially rigid substrate may be integrallyformed into a single piece. The antenna may be supported by said holder.The holder may include a stop that limits the movement of the integratedchip into the heart chamber. The integrated chip and the holder may beremovable from the heart after implantation. The at least one pressuresensor may include a plurality of pressure sensors including at leastone active sensor responsive to changes in pressure within the heart andat least one passive sensor that is isolated from the changes inpressure within the heart, and the electronic signal processingcomponents may provide a signal based at least in part on a signal fromthe at least one active pressure sensor and a signal from the at leastone passive pressure sensor. The structure of the active pressure sensormay be substantially the same as a structure of the passive pressuresensor. The plurality of pressure sensors may include capacitivepressure sensors each having a flexible movable membrane. The passivepressure sensor signal may be responsive to a change in position of themembrane of the passive pressure sensor, which is due to a drift effectcomprising a sag of said membrane. The change of position of themembrane of the active pressure sensor may be due to a change inpressure within the heart and a drift effect comprising a sag of themembrane. The pressure signals may be the result of offsetting thesignal from the at least one active pressure sensor with the signal fromthe at least one passive pressure sensor.

According to another aspect of the invention a method of sensing bloodpressure within the cardiovascular system of a subject includes thesteps of (a) implanting within the subject an integrated chip includinga substantially rigid substrate and at least one capacitive-basedpressure sensor disposed within said substrate in a position to senseblood pressure within the cardiovascular system; (b) powering on theintegrated chip telemetrically by activating a power source locatedoutside the subject; (c) obtaining one or more analog signals from theat least one pressure sensor indicative of the pressure at the positionin the cardiovascular system; and (d) converting the analog signals todigital signals at or directly adjacent to the position in thecardiovascular system where the sensing occurs.

The implanting step may include implanting an ASIC having acapacitive-based pressure sensor in the heart. The method may furtherinclude the step of limiting the ASIC from entering the heart chamberwith a stop device. The integrated chip may include a unique digitalidentification, and the method may include the step of telemetricallycommunicating the unique digital identification to an external reader.The method may further include the step of obtaining calibrationinformation associated with the integrated chip at the external readerbased on the unique digital identification. The integrated chip may besupported in a holder and the implanting step may include the steps ofdelivering the holder to the position in the cardiovascular system, andmounting the holder at the position such that the at least one pressuresensor is exposed to the pressure in the cardiovascular system to besensed.

The at least one capacitive-based pressure sensor may include aplurality of capacitive-based pressure sensors including an activepressure sensor and a passive pressure sensor located within the subjectin a position to directly sense blood-pressure within a position in thecardiovascular system, and the step of obtaining one or more analogsignals may further include obtaining one or more analog signals fromthe active pressure sensor indicative of the pressure at the position inthe cardiovascular system, obtaining one or more analog signals from thepassive pressure sensor indicative of the pressure at the position inthe cardiovascular system, and generating one or more combined analogsignals based on the one or more analog signals from the active pressuresensor and the one or more analog signals from the passive pressuresensor indicative of the pressure at the position in the cardiovascularsystem. The step of converting the analog signals may further includeconverting the combined analog signals to digital signals. The step ofgenerating the one or more combined analog signals may includeoffsetting the signal from said active pressure sensor with the signalfrom said passive pressure sensor.

In yet another aspect of the invention an integrated chip forintra-cardiac blood pressure measurement inside the heart of a patientincludes a first substantially rigid substrate, at least one pressuresensor disposed within the substrate to generate signals indicative of asensed pressure, and electronic signal processing components to processthe signals generated by the at least one pressure sensor. Theelectronic signal processing components may be operatively connected toan antenna, and the integrated chip may be powered by a signal receivedat the antenna. The integrated chip is operative to send digital signalsindicative of the pressure sensed in the heart telemetrically via anantenna to a remote receiver.

The at least one pressure sensor may generate analog signals and theelectronic signal processing components may include at least one analogto digital (A/D) converter to digitize within the heart the analogsignals from the at least one pressure sensor. The integrated chip mayweigh less than about one gram, have a surface area on one side of lessthan or equal to about 10 mm² and have a thickness of less than about 1mm. The integrated chip may further include a second substantially rigidsubstrate located opposite the at least one pressure sensor in the firstsubstrate and in a spaced apart configuration. The second substrate mayinclude an aperture permitting blood pressure within the heart to act onthe at least one pressure one pressure sensor. The integrated chip mayalso include a pressure transferring medium interposed between the atleast one pressure sensor and the second substantially rigid substrateto transfer blood pressure to the at least one pressure sensor.

Additional features, advantages, and embodiments of the invention may beset forth or apparent from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are exemplary and intended to provide further explanationwithout limiting the scope of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the detailed description serve to explain the principlesof the invention. No attempt is made to show structural details of theinvention in more detail than may be necessary for a fundamentalunderstanding of the invention and the various ways in which it may bepracticed. In the drawings:

FIG. 1 schematically illustrates an embodiment of an implantabletelemetric measuring device and reader constructed according toprinciples of the invention providing for continuous or regularintra-cardiac pressure monitoring;

FIG. 2 schematically illustrates another embodiment of an implantabletelemetric measuring device and reader constructed according toprinciples of the invention providing for on-demand intra-cardiacpressure monitor monitoring;

FIG. 3 illustrates a cross-sectional view of the heart area of a patientwhere the implanted device of the invention may be employed, includingthe left and right atrium and the crossing of veins at the posteriorseptum;

FIG. 4 illustrates a greatly enlarged, plan view of a substantiallyrigid ASIC constructed according to principles of the invention forsensing intra-cardiac pressure and temperature including active andpassive, capacitive membrane sensing elements and on-chip electronicsfor digital signal processing and telemetrical power supply.

FIG. 5 is an enlarged, cross-sectional view of the ASIC of the inventionshowing some of the active pressure sensors and passive pressuresensors;

FIG. 6 is an enlarged, cross-sectional view of the ASIC of the inventionshowing a pressure transmitting gel or fluid between a sheathing and theactive pressure sensors;

FIGS. 7 and 8 schematically illustrate a cross-sectional and top planview, respectively, of one embodiment of an implantable sensor device ofthe invention including a substantially rigid ASIC connected at two endsto a substantially rigid substrate having a cut out;

FIGS. 9 and 10 schematically illustrate a cross-sectional and top planview, respectively, of another embodiment of an implantable sensordevice of the invention including a substantially rigid ASIC connectedat one end to a substrate having a cut out;

FIG. 11 is a perspective illustration of the implantable device of FIGS.7 and 8 or 9 and 10, showing an electrical wire and filament coreconnection between the ASIC and the antenna;

FIG. 12 illustrates a perspective view of the implantable sensor deviceof the invention with a cut out located at an edge of the substrate,

FIG. 13 is a perspective view showing the electrical wire and filamentcore connection of the FIG. 12 embodiment;

FIG. 14 is a cross-sectional view that schematically illustrates theelectrical wire and filament core connection to a substrate of theinvention;

FIGS. 15 and 16 schematically illustrate yet another embodiment of animplantable sensor device of the invention having a cut out located atan edge of the substantially rigid substrate and a protective barrierwall located at one end;

FIG. 17 schematically illustrates a cross section view of a furtherembodiment of the protective barrier wall of the invention;

FIG. 18 schematically illustrates an integrally formed ASIC and holderof the invention implanted in a wall of the heart during surgery;

FIG. 19 schematically illustrates another integral ASIC and holder ofthe invention having a “T”-shaped anchor implanted in a wall of theheart during surgery;

FIG. 20 schematically illustrates an ASIC and separately formed holderof the invention in the form of an elbow connector holding the ASIC andguiding a slack portion of the connecting wire;

FIG. 21 is a three-dimensional representation of a holder of theinvention that mounts the ASIC in the wall of the heart and includessuture wings that limit movement of the ASIC and serve as suture mounts;

FIG. 22 is a three-dimensional representation of another embodiment of aholder of the invention where the holder includes an end cap;

FIG. 23 is a three-dimensional representation of a holder of theinvention illustrating how it connects the ASIC via a flexible cable toan antenna;

FIG. 24 schematically illustrates an implantable sensor device of theinvention encased in a biocompatible sheathing;

FIG. 25 is a side view of the device illustrated in FIG. 24 illustratinghow the shape of the sensor may be configured as a football shape tominimize turbulence and reactionary fluid forces in the heart;

FIG. 26 is a side view of a dual substrate sensor device of theinvention showing how the shape of the sheathing may be configured tominimize turbulence and reactionary fluid forces in the heart;

FIG. 27 is a side view of another embodiment of the inventionillustrating how the shape of the sheathing may be configured tominimize turbulence and reactionary fluid forces in the heart; and

FIG. 28 is a block diagram of the major electronic components of anexternal reader constructed according to the principles of the inventionfor telemetrically receiving data from an implanted sensor device.

DETAILED DESCRIPTION

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments and examples that are described and/orillustrated in the accompanying drawings and detailed in the followingdescription. It should be noted that the features illustrated in thedrawings are not necessarily drawn to scale, and features of oneembodiment may be employed with other embodiments as the skilled artisanwould recognize, even if not explicitly stated herein. Descriptions ofwell-known components and processing techniques may be omitted so as tonot unnecessarily obscure the embodiments of the invention. The examplesused herein are intended merely to facilitate an understanding of waysin which the invention may be practiced and to further enable those ofskill in the art to practice the embodiments of the invention.Accordingly, the examples and embodiments herein should not be construedas limiting the scope of the invention, which is defined solely by theappended claims and applicable law. Moreover, it is noted that likereference numerals reference similar parts throughout the several viewsof the drawings.

FIG. 1 schematically illustrates an embodiment of an implantabletelemetric measuring device and reader constructed according toprinciples of the invention providing for continuous or regularintra-cardiac pressure monitoring. A coil or antenna 14 connected withan external reader 12 generates a radio frequency (RF) field in a mannerknown in the art. The coil 14 and external reader 12 may be fixed toindividual belts that wrap around the patient and connect to each othervia a standard cable. The RF field induces a current in a coil 18connected to a substantially rigid sensor device 20, such as describedherein, implanted within the heart 16 of the patient 10, such as theseptum or the wall of the left atrium, to sense pressure in the leftatrium. The sensor device 20 may consist of an application specificintegrated circuit (ASIC) such as described herein, having powerconditioning circuitry that detects when adequate power is beingdelivered and switches on sensing, analog-to-digital, and dataprocessing circuits. The data processing circuitry sends the sensor datato the ASIC transmitter, which uses the coil 18 as an antenna. The coil18 telemetrically transmits, via signal 22, the data to the antenna 14of the external reader 12. The external reader 12 may provide securereception and storage of pressure and temperature values, compare thepressure reading of the implanted device 20 to ambient pressure via aninternal sensor in the reader, and deliver the intracardiac data toother devices, such as computers, personal digital assistants (PDAs),cell phones, etc., via standard protocols.

In this embodiment, the external reader 12 may obtain data from thesensor device 20 at continuous or regular intervals. By way of example,the external reader 12 may continuously generate an RF signal toactivate the sensor device 20 to obtain pressure and/or temperaturereadings (in order to describe even the waveform of the blood pressure,if desired by the doctor, the sensor device should take up to 100 ormore measurements per second). Alternatively, the external reader 12 maygenerate an RF signal at regular intervals (e.g., every half hour, onceever four hours, once a day) to activate the implanted rigid sensordevice 20 to obtain pressure and/or temperature readings.

FIG. 2 schematically illustrates another embodiment of an implantabletelemetric measuring device and reader, which may operate similarly tothe FIG. 1 embodiment but provides for on demand intra-cardiac pressuremonitoring according to principles of the invention. In this embodiment,a coil 28 in a hand-held reader 26 generates an RF field that induces acurrent in the coil 18 of the substantially rigid sensor device 20implanted within the heart 16 of the patient 10, as in the FIG. 1embodiment. As described above, sensor device 20 may include an ASICthat operates similarly to the FIG. 1 embodiment. Thus, powerconditioning circuitry in the sensor device 20 detects when adequatepower is being delivered, and turns on the sensing, analog-to-digital,and data processing circuits. The data processing circuitry sends thesensor data to the ASIC transmitter, which uses the coil 18 as anantenna. The coil 18 transmits, via signal 24, the data to the antenna28 of the hand-held reader 26. The hand-held reader 26 may be extendableto expose the antenna 28 and provides reception and storage of pressureand temperature values, and compares the implant's pressure reading toambient pressure via an internal sensor in the readout device. Thehand-held device 26 may deliver the intracardiac data to other devices,such as computers, PDAs, cell phones, etc., via standard protocols.

In this embodiment, the reader unit 26 may obtain data from theimplanted sensor device 20 on demand. By way of example, a user mayactivate and cause the reader unit 20 to generate an RF signal byextending the top portion containing the antenna from the bottom portionof the reader unit housing to activate the implanted rigid sensor device20 to obtain pressure and/or temperature readings.

FIG. 3 illustrates a cross-sectional view of the heart area of a patientwhere the implanted device may be employed, including the left and rightatrium and the crossing of veins at the posterior septum. The heart 30has a right atrium 32 and a left atrium 34, which are divided by theseptum 36. As described in more detail herein, it may be advantageous tolocate and/or anchor the implantable sensor device 20 at the septum 36separating the right atrium 32 and the left atrium 34, such that aportion of the sensor 20 extends into the chamber to be sensed, e.g.,the left atrium 34. The implantable device may work as a short-termimplant as well as a long-term implant, and may be implanted at the“Waterston's groove” near the access of the pulmonary vein or otherlocations chosen by a doctor. The implantable sensor device 20 also maybe designed to facilitate ready removal of the device if medicallynecessary. An embodiment of such a removable device is illustrated inFIG. 23.

FIG. 4 illustrates a substantially rigid sensor ASIC constructedaccording to principles of the invention for sensing intra-cardiacpressure and temperature in any of the embodiments of the invention. TheASIC 400 contains pressure sensing elements 402, such as eight passivesensors 404 and eight active sensors 406, temperature sensor 408, ananalog-to-digital (A/D) converter 410, data transmission circuitry 412,power conditioning circuitry including components such as smoothing andresonance capacitors (not shown), a digital state control 414 includinga code redundancy check for secure data transmission and memory 416,such as Electrically Erasable Read-Only Memory (EEPROM) cells, for unitidentification, which are components known in the cart. An example ofthe suitable ASIC structure is described in U.S. Pat. Nos. 5,321,989 and5,431,057, the contents of which are expressly incorporated by referencein their entirety.

According to the principles of the invention, the ASIC 400 should be anextremely small and lightweight chip to avoid placing undue stress onthe heart and/or producing turbulent flow in the heart chamber(s). Forexample, an ASIC particularly adapted for use in the embodimentsdescribed herein as being implanted during open chest surgery shouldweigh less than fractions of a gram, have a surface area of less than orequal to about 10 mm² per side, and a thickness of about ¼ mm to about 1mm. In one advantageous embodiment, the ASIC may be about 2 mm wide by 5mm long by about 250 to 800 microns thick. Other dimensions may also beused depending upon the particular application or location in thecardiovasculature where the sensing will occur and depending upon thedelivery method. In general, the dimensions of the ASIC 400 may rangefrom about 3 mm to about 8 mm long, about 0.6 mm to about 2.5 mm wide,and about 0.2 mm to about 1.3 mm high. Other dimensions, such as an ASICthat is substantially square, may also be used.

In the embodiment of the invention shown in FIG. 4, the ASIC 400includes sixteen capacitive pressure sensors cells 402, eight of whichare active pressure sensors 406 and provide pressure data, and eight ofwhich are passive pressure sensors 404 and act as an internal reference.The pressure sensor cells 402 may include minute, flexible membranesthat are housed within the substantially rigid ASIC structure as shownschematically in FIG. 5. Specifically, the active pressure sensors 406have flexible membranes 424 and passive pressure sensors 404 haveflexible membranes 426. The membranes 424 of the active pressure sensors406 are distortable based on the level of cardiac blood pressure. Thedistortion may be mainly in a direction generally perpendicular to theplanar top surface of the ASIC 400. The distortion may be determinedbased on capacitive measurements or by use of distension measuringtapes. By way of one specific exemplary embodiment, the sixteencapacitive pressure sensing elements 402 of the ASIC 400 may each beabout 96 microns in diameter.

As the pressure changes in the heart, the capacitance measured in thepressure sensors 402 changes. The pressure sensors 402 generate signalsbased on the change in capacitance, and thus indicative of the pressurein the heart. As will be described below, the signals preferably areprocessed by components located in or on the ASIC 400 and transmitted toan external reader.

Thus, the blood pressure measuring process may be a capacitive pressuremeasurement process via measuring membranes 424, 426 that are integratedinto the ASIC 400, such as the planar top surface of the chip as shownin FIG. 5. The ASIC 400 may have a substantially inflexible, substratemade of silicon that cannot be folded or rolled up. The thin, butmechanically inflexible substrate creates a mechanically stable deviceproviding a substantially rigid structure to house the measuringmembranes 424, 426 as shown in FIG. 5. Changes in the geometry of theASIC 400, such as twisting due to blood turbulences, may be avoided dueto this substantially rigid, chip-based configuration, even when theASIC 400 is exposed to turbulent, blood flow. Thus, the implanted ASIC400 provides a durable device capable of withstanding the internalenvironment of the heart and other locations in the cardiovasculaturewithout producing dangerous stresses within the heart.

Numerous small membranes 424, 426 having relatively small dimensions(e.g., a diameter of less than 0.2 mm) may be used as capacitivepressure sensors. Such small dimensions may result in membranes 424 thatare less vulnerable to mechanical forces, such as the force of bloodflow within the heart, and therefore more reliable.

The ASIC 400 contains mechanical and electrical elements that aresubject to wear and need drift compensation to obtain measurements ofsuitable quality and reliability for their intended cardiovascular uses.Drift in a sensor may occur as time passes and physical properties ofthe structure change. Over time and usage, changes in electronics in achip may effect the measurements. Further, when a pressure sensor uses amembrane, the membrane may sag in the middle as it ages. The capacitanceat the pressure sensor membrane varies based on the change in positionof the pressure sensor membrane. These changes, unrelated to the changein blood pressure, may alter the true value of the measurements beingsensed. Drift compensation is particularly important in an intra-cardiaclong-term pressure sensor.

The drift compensation scheme employed in ASIC 400 should reduce oreliminate the effects of the change in the physical properties of theASIC 400. According to the principles of the invention, the drift of thepressure values obtained from the sensor structure 400 may be minimizedto a value of about 5.0 mmHg/year to about 2.5 mmHg/year or even smallerthan 1 mmHg/year.

In accordance with drift compensation principles of the invention, aplurality of active sensors 406 and a plurality of passive sensors 404are provided, such as eight of each. According to an embodiment of theinvention, the structure of the active sensors 406 and the structure ofthe passive sensors 404 are identical. However, as illustrated in FIG.5, the membranes 424 of the active sensors 406 are open to the sensingenvironment (e.g., a heart chamber) for sensing pressure, while themembranes 426 of the passive sensors 404 are isolated from theenvironment, e.g., by placing a glass layer 428 or other suitablematerial over the surface of the membrane 426 so that pressure in heartwill not affect the passive sensors 404. Both the active sensors 406 andpassive sensors 404 are affected substantially the same by age, usageand sagging and any other effects of the environment. Using the passivesensors 404, the ASIC 400 may determine how much of the change inposition of the pressure sensor membrane 426 is effected by the age andsagging. The change in capacitance based on the change in position ofthe passive pressure sensor membrane 426 is determined. This amount isthen used to offset the change capacitance measured in the activepressure sensor membrane 424. This system allows the change incapacitance due the pressure within the heart to be more accuratelydetermined. Compensating for drift may allow a doctor or patient tobetter determine short term (e.g., days, weeks) trends in pressurewithin the patient, such as the heart.

The implantable sensor device, which may include the ASIC 400, aconnector and an antenna, may be completely encapsulated within aseamless biocompatible sheathing (not shown in FIGS. 5-6). The materialareas around the measuring membranes 424 maintain their flexibilityafter encapsulation to allow transmission of the pressure to themeasuring membranes 424. The biocompatible sheathing will be describedin greater detail below.

FIG. 6 is a cross-sectional view of the ASIC 402 of the invention with agel or fluid between a sheathing and the active pressure sensors 406. Asdescribed above, a glass substrate 428 or other suitable materialisolates the passive pressure sensors 404. A liquid or gelatinouspressure transmitting medium 432 is used between sheathing 430 and theactive pressure sensors 406. As will be described below, this liquid orgelatinous medium 432 may improve the measurement or reception of bloodpressure values within the chamber to be sensed, e.g., within the leftatrium. Even though fibrous tissue or plaque may grow in the area of theimplant over time (e.g., months or years after the implantation),encapsulating the pressure sensors within a separate gel-filled membranemay allow reliable measurement values to still be obtained.

For example, endothelialization may result in endothelia being depositedon the surface of the sensor structure. If endothelia and/or plaque aredeposited on the surface of one of the active pressure sensors, or onthe biocompatible sheathing at the surface of one of the active pressuresensors, pressure measurement readings may be adversely affected. Oneway to reduce such an effect is to coat the sheathing and/or sensorswith a drug, e.g., heparin, to reduce or eliminate endothelia. However,such treatments may not always be effective.

Thus, as illustrated in FIG. 6, the surface of the active pressuresensors 406 are coated with a gel or fluid 432 and encapsulated in themembrane 430. In this manner, endothelial growth or plaque on themembrane 430 directly over the surface of one of the active pressuresensors 406 will have a reduced or negligible effect on the pressuresensor measurement, as the pressure is transmitted via the endothelialgrowth and the membrane 430 through the gel/fluid 432 to the activepressure sensors 406. Further, plaque growth and/or endothelializationon the entire surface would still allow pressure sensing measurements tobe obtained, as the pressure exerted on the endothelia is transmittedvia the gel/fluid 432 to the active pressure sensors 406. In particular,the gel/fluid filled membrane 430 may function to integrate the changein pressure over a larger area than the individual active pressuresensors 406 themselves. This minimizes the effects of endothelializationand/or plaque adherence to the sheathing 430. Although sheathing 430 isshown as only covering the gel/fluid 432, it is understood that thesheathing 430 or other sheathings could cover part or all of sensordevice 400, as described below.

As described above, the ASIC 400 includes an A/D converter 410. As isknown in the art, the pressure sensors 402 provide analog signalsindicative of the pressure in the heart. The A/D converter 410 convertsthe signals from the pressure sensors 402 to digital signals.

Thus, the transmission and digitizing of measurement values intoappropriate signals in the invention is preferably carried out within orvery closely adjacent to the heart chamber or chambers to be sensed,such as the left and/or right atrium and/or the left or right ventricle,and most preferably are processed inside the ASIC 400. Using a fullydigital system may result in greater accuracy of the readout. In ananalog system, where the amplitude of the signal is proportional to thepressure reading provided by the sensors, the value of pressure recordedby an external reader depends upon the distance between body and reader.As a result, either the distance from the body to reader must be verytightly controlled, or the accuracy of the system will suffer. Accordingto the invention, the distance from body to reader has little or noeffect on the pressure value measurement received due to the use of adigital signal and to processing the signals at or very near the sensor.This may make the system more robust and accurate than analog systems.

In addition, the fully digitized data can be handled for more easily bydata transmission systems, making the external readers compatible withcomputer, Internet and telemedicine interfaces. For example, highlyaccurate pressure sensors and a 9-bit analog-to-digital converter mayimpart high resolution to the sensing systems, where an accuracy ofabout +/−2 mm Hg or less may be achieved.

Further, digitization at the ASIC 400, as opposed to analog signaltransmission via an antenna before digitization, may avoid interferenceissues from other, unrelated RF sources. In prior devices, analogsignals are sent from the sensor to the antenna structure via a wire. Byprocessing and converting the analog signals to digital signals prior totransmission over the wire to the antenna, the system may avoid analoginterference that may be induced in the wire by external RF signals andnoise, such as radio broadcasts, electronics, and the like.

The ASIC 400 measures pressure at the pressure sensing elements 402 andtransfers the absolute pressure signals to an external reader. Apressure value is calculated from the difference of absolute pressurevalue, measured with the ASIC 400, and the atmospheric pressuresurrounding the patient as is well-known in the art. This atmosphericpressure may be measured within the external reader, which is normallyin the surrounding environment of the patient.

The operation of the ASIC 400 is based on the interaction between aconnected antenna, such as shown in FIG. 23, and an external readeraccording to well-known principles of transponder technology. Therefore,no internal power source is required. The ASIC 400 and the externalreader may be tuned so that continuous measurements, e.g. up to 120single measurements per second, may be processed and transmitted. Asdescribed above in FIG. 1, the total system may be programmed so thatmeasurements are taken and stored in given intervals or at defined timeperiods. Retrieval, monitoring, and recording of data may be possible atany time.

According to an embodiment of the invention, the ASIC 400 preferablyconsists of a single integrated chip. All relevant functions andcomponents for the measuring process, digitizing, identification numbertransmission, power supply, and telemetric data transmission areintegrated into the single integrated chip. As described above, the ASIC400 may contain a specific identification number, as well as a chipspecific calibration file and further circuit and storage components.Alternatively, the circuit components may also be placed on two or morechips, e.g. if sensing in separate locations is desired.

The ASIC 400 may be formed from a single complementary metal oxidesemiconductor (“CMOS”) chip to produce a smaller implantable device thenwith other methods, and help minimize power use and maximize measurementaccuracy reliability. Since the consumption of power produces heat,minimization of power may be desirable in implantation applications. Ina one-chip solution, the ASIC 400 may be highly resistant to mechanicalor electrical interference from the outside, as there is no interactionbetween multiple chips.

The power consumption of the chip may be low, so that if an increase oftemperature occurs in the course of inductive/transponder related powerinsertion, difficulty in measuring or data transmission may be reducedor avoided. The optimized circuit design may result in a very low powerconsumption, such as only about 210 microwatts at about 3 volts DC. Thesampling rate may be about 20 to about 120 Hz. The high integrationfactor of the logic circuit combined with the high speed of datatransmission may allow the use of a very secure data transmissionprotocol, thereby addressing concerns of the regulatory authorities.

An integrated temperature sensor 408 may be provided in the ASIC 400 toallow for temperature sensing as shown in FIG. 4. The temperature sensor408 may use the circuit in the ASIC 400 and base the temperaturemeasurement on current characteristics within the circuit, therebydetermining the temperature in the heart based on the temperature basedcurrent characteristics within the ASIC 400. Each ASIC 400 may beindividually calibrated to determine its current characteristics(magnitude, frequency, etc.) at a given temperature (e.g., bodytemperature). As the temperature changes, the current characteristicswithin the ASIC 400 change. Using the information on the currentcharacteristics and the specific calibration determination for the ASIC400, the temperature at a particular time can be determined based oncurrent characteristics at that time. The raw pressure data must becorrected for temperature and other external and/or internal influences,and calibration information, such as a calibration curve of the embeddedchip, may be established for each ASIC 400 or system that implements anASIC 400. Each ASIC may have a unique identification number tofacilitate calibration and use of data as discussed below.

The ASCI 400 includes a data memory 416, such as the EEPROM cells, inwhich the unique identification number may be stored. Thisidentification number is transmitted telemetrically together with themeasurement values. The identification number may be used to determinethe appropriate calibration information for an ASIC 400. Also, a singleexternal reader may then be used to interrogate multiple implantedASICs, as described below.

The unique identification number may be transmitted along with thesensor data to the external reader to allow the external reader to usethe correct calibration information to calculate pressure and/ortemperature. An external reader (as described in greater detail below),may have a memory to store calibration information for a number of ASICs400 or systems that implement ASICs 400. The appropriate calibrationinformation is associated the appropriate ASIC 400 or system via theidentification number. With the identification number, or otheridentification indicia, the external reader accesses the calibrationinformation associated with the particular ASIC 400 or system thatincludes the particular ASIC 400. The data received by the externalreader is processed using the appropriate calibration information toachieve more accurate results.

Each ASIC 400 and/or system also may be zeroed prior to implantation.When inside the patient, the system compares the measured pressure tothe pressure in a vacuum. Outside the patient, the external readercompares the ambient pressure to the pressure in a vacuum. Pressureinside the heart is calculated by comparing the difference between thepressure measured inside the heart and the pressure measured outside thepatient. Zeroing the ASIC 400 or the system may involve using the ASIC400 system to measure the pressure outside the patient and comparingthis measurement to the pressure obtained by another external device.The difference between these two readings may be stored with thecalibration information associated with the ASIC 400 or system and usedto adjust future pressure measurements by the ASIC 400 or system once ithas been implanted to account for the difference.

Using one or more transponder coils, an external reader may be used forthe power supply of the ASIC. This unit also may be used for telemetricdata acquisition. The range for telemetric power supply and datatransmission may be from about 3 cm to about 35 cm or other ranges ascan be readily determined by a skilled artisan. This range also maydepend on the distance between the external reader and the implantedantenna and the size of the antennas.

Measurement data are processed and preferably are digitized on the ASIC400 for transmission from the sensor structure to the interiortransponder coil. The transmission of the measurement data from the ASIC400 to the interior transponder coil may be realized via one or moreelectric conductors, preferably designed as flexible thin wires,embedded in silicone on other nonconducting material. Measurement dataare transmitted telemetrically from the interior transponder coil to theexternal reader. The external reader capacities may be designed for anexterior supply of all power resources which are required for thecontinuous operation of ASIC 400, including measurements and datatransmission.

The ASIC 400 also includes a bi-directional power circuitry 424 forworking with the reader to evaluate the strength of the signals sentbetween the reader and the ASIC 400. The components in thebi-directional power circuitry 424 interact with a reader to ensure thatappropriate signal strength and data transmission is achieved. Theinteraction between the bi-directional power evaluation module 424 andthe reader is described in greater detail below with respect to FIG. 28.

FIGS. 7 and 8 schematically illustrate an embodiment of an implantablesensor device 700 of the invention including a substantially rigidsensor chip connected at two ends to a substantially rigid substrate 708having a cut out. A sensor chip 702, such as ASCI 400, includes pressuresensing membranes 704 and four spaced chip bond pads 706. Asubstantially rigid substrate 708 having an aperture 710 and bond tracks712 connected to bond pads 706 are also provided. The substrate 708 isconfigured in a spaced apart relationship to the sensor chip 702. Moreparticularly, the aperture 710 of the substrate 708 is locatedsubstantially opposite of the capacitive pressure membranes 704 of thesensor chip 702 so pressure from the blood surrounding the device may betransmitted readily to the pressure membranes 704. A pressuretransferring material (not shown) may be located at the aperture 710 toensure that pressure from the blood is transferred to the pressuremembranes 704.

The sensor chip 702 and the substrate 708 may be configured in a fixedrelationship, so that the distance, or offset, between the sensor chip702 and the substrate 708 does not change. The chip bond pads 706 may beconnected to the substrate bond pads 712 to fix the distance between thesensor chip 702 and the substrate 708. As shown in the embodiment ofFIGS. 7 and 8, the sensor chip 702 and the substrate 708 both have fourbond pads. However, it is understood that other amounts of bond pads mayalso be used.

At least one of the substrate bond pads 712 may be elongated in the formof a track to facilitate connection to an electrical wire 714 thatconnects to an antenna (not shown). Electrical wire 714 is connected tothe substrate bond pad 712 by any conventional method, such as by usingheat and pressure. Connecting the electrical wire 714 to a substratebond pad 712, as opposed to being directly connected to chip 702, mayreduce or eliminate damage to or malfunction by the sensor chip due tothe connection process. The electrical wire 714 is electricallyconnected to the sensor chip 702 via the electrical connection betweenthe substrate bond pad 712 and the chip bond pad 706.

FIGS. 9 and 10 schematically illustrate another embodiment of animplantable sensor device including a substantially rigid sensor chipconnected at one end to a substantially rigid substrate having a cutout. The device 900 of FIGS. 9 and 10 has similar components andoperation to the device 700 illustrated in FIGS. 7 and 8. However,device 900 has chip bond pads 906 located in generally close proximityto each other at one end of the sensor chip 902. In addition, thesubstrate bond pads 912 are generally located in close proximity to eachother on the substrate 908. When the chip bond pads 906 and thesubstrate bond pads 912 are connected, the sensor chip 902 and thesubstrate 908 are fixed at one end, with the other free end beingsupported in a cantilevered manner. This arrangement of chip bond pads906 and substrate bond pads 912 may reduce stress on the sensor chip902, as changes in the size of the substantially rigid substrate 908,such as due to thermal expansion, may have less of an effect on thesensor chip 902 due to the location of the chip bond pads 906 on thesensor chip 902.

The device 900 may further include a flexible filler material 916located between the sensor chip 902 and the substrate 908. As shown, thefiller 916 may be located throughout the area between the sensor chip902 and the substrate 908 except at the aperture 910 that is oppositethe capacitive pressure membranes 904. Filler 916 may be any flexiblematerial that can provide support to reduce or eliminate movement in theoffset direction between the sensor chip 902 and the substrate 908. Thefiller 916 may be the same material used to surround the implanteddevice 900, such as a biocompatible material like silicone or othersimilar material.

FIG. 11 is a perspective illustration of an implantable sensor devicesuch as the FIGS. 7 and 8 or FIGS. 9 and 10 embodiments showing theelectrical wire and core filament connection to the ASIC and antenna.The device 1100 includes a substantially rigid sensor chip 1102 havingpressure membranes 1104, and a substantially rigid substrate 1108 withan aperture 1110 exposing the pressure membranes 1104. A pressuretransmitting material 1112, such as a liquid or gelatinous material, islocated within the aperture 1110 to transmit pressure from the blood tothe pressure membranes 1104. The entire device 1100 is enclosed by abiocompatible sheathing 1106, such as silicone. In addition, thesheathing 1106 can also be used as the pressure transmitting material1112 within the aperture 1110.

Substrate 1108 may further include connector holes 1120 for facilitatingattachment of an antenna connector 1114 to the substrate 1108 and thesensor chip 1102. The connector 1114 includes electrical wires 1116 anda filament core 1118, such as nylon. Electrical wires 1116, which may beformed of gold cable, or other appropriate material, provide anelectrical connection between the sensor chip 1102 and an antenna (notshown). Electrical power from the antenna may be conducted via theelectrical wires 1116 to the sensor chip 1102 for powering the sensorchip 1102 to obtain measurements. Signals, such as pressure measurementsand identification indicia, may be transmitted over the electrical wires1116 from the sensor chip 1102 to the antenna for transmission to areader. The filament core 1118 provides strength to the connector 1114to reduce or eliminate strain on the connection between the substratebond pad (not shown) and the electrical wires 1116. The filament coremay be made of nylon or other similar, synthetic flexible material thatdoes not conduct electricity and has a low coefficient of thermalexpansion. This connection will now be described in greater detail belowwith reference to the examples of FIGS. 12-14.

FIGS. 12 and 13 illustrate an implantable sensor device 1200 with a cutout located at an edge of the substantially rigid substrate, including acable and core filament connection, while FIG. 14 schematicallyillustrates the cable and core filament connection to the substrate. Theimplantable device 1200 includes a substantially rigid sensor chip 1202having pressure membranes 1204. In this embodiment, the capacitivepressure membranes 1204 are located near the edge of one side of thesensor chip 1202. The device 1200 further includes a substantially rigidsubstrate 1208 having connector holes 1220 and a cut out 1210 oppositeof the pressure membranes 1204. A pressure transmitting material 1212 islocated within the cut out 1210 to transmit pressure from the blood tothe pressure membranes 1204. The device 1200 is surrounded by abiocompatible sheathing 1206, such as silicone. According to a preferredembodiment of the invention, the pressure transmitting material 1212 maybe the same as the sheathing material 1206.

The device 1200 further includes a connector 1214 which includeselectrical wires 1216 and a filament core 1218. The electrical wires1216, which may be formed of gold, or any other suitable similarmaterial, connect to substrate bond pads 1222, and the substrate bondpads 1222 are connected to chip bond pads 1224. This results in anelectrical connection between the electrical wires 1216 and the sensorchip 1202. The filament core 1218 may be attached directly to thesubstrate 1208, such as by an adhesive. As shown in FIG. 14, thefilament core 1218 is threaded through the connector hole 1220 forattachment to the substrate 1208 such that the electrical wires 1216have extra slack when the filament core 1218 is pulled straight. Thisconfiguration may reduce or eliminate the strain on the connectionbetween the electrical wires 1216 and the substrate bond pad 1222 whenthere is movement of either the connector 1214 or the substrate 1208.Other methods for strain relief on electrical wires 1216 may also beused.

FIGS. 15 and 16 schematically illustrate an implantable sensor devicewith a cut out located at an edge of the substrate and a protectivebarrier wall located at one end of the substrate. The device 1500 has asubstantially rigid sensor chip 1502 having capacitive pressuremembranes 1504 (shown in FIG. 16). The device 1500 further includes asubstantially rigid substrate 1508 having a cut out 1510 locatedsubstantially opposite the pressure membranes 1504. A chip bond pad 1518on the sensor chip 1502 is connected to a substrate bond pad 1520 of thesubstrate 1508 in a conventional manner. The device 1500 is encapsulatedin a biocompatible sheathing 1506.

The substrate 1508 includes a barrier wall 1514 that may besubstantially perpendicular to the plane of the substrate 1508. Theheight of the barrier wall 1514 may be such that the top of the barrierwall 1514 is at or above the top of the sensor chip 1502 when it isattached to the substrate 1508. The barrier wall 1514 may provideadditional protection to the chip sensor 1502, such as preventing thesharp ends of the chip 1502 from wearing or puncturing the sheathing1506. In addition, a front portion 1512 of the substrate 1508 shapedlike an arrow is located beyond the barrier wall 1514 and is tapered toreduce or eliminate the effects of blood turbulence on the chip sensor1502, as well as aid in the implantation of the device 1500 within theheart. This may occur when the tapered portion 1512, and thus the device1500, is inserted into the heart. The edges of the barrier walls may beslightly rounded (not shown in the drawings) to avoid any wearing orpuncturing of the sheathing. Although not shown, it is understood that apressure transmitting material and/or a filler material may be used withthe device 1500.

FIG. 17 schematically illustrates a cross sectional view of a furtherembodiment of a barrier wall formed as an end cap at one end of thesensor device. The device 1700 has a sensor chip 1702 having capacitivepressure membranes 1704. The device 1700 further includes a substrate1708 having a cut out 1710 located substantially opposite the pressuremembranes 1704. A chip bond pad 1718 on the sensor chip 1702 isconnected to a substrate bond pad 1720 of the substrate 1708 in aconventional manner. The device 1700 may also be encapsulated in abiocompatible sheathing (not shown in FIG. 17).

The substrate 1708 may include a barrier wall 1714 that is substantiallyperpendicular to the plane of the substrate 1708, as in the priorembodiment. The height of the barrier wall 1514 may be such that the topof the barrier wall 1714 is at or above the top of the sensor chip 1702when it is attached to the substrate 1708. In addition, the barrier wallincludes a top cover 1716 extending inwardly from the top of the barrierwall 1714 substantially parallel to the substrate 1708 over the sensorchip 1702 to provide protection to the top of the sensor chip 1702.Although shown in FIG. 17 as extending over only a small area of thesensor chip 1702, it is understood that the barrier top cover 1716 couldextend further, including along the entire length of the sensor chip orbeyond. The barrier wall 1714 and the barrier top cover 1716 may provideadditional protection to the chip sensor 1702 and the sheathing asdiscussed above. More specifically, the barrier top cover 1716 mayinhibit damage to the sheathing 1706 that could occur by rubbing ofsharp edges of the chip sensor 1702 against the sheathing material 1706.In addition, a front portion 1712 of the substrate 1708 is locatedbeyond the barrier wall 1714 and may be tapered to reduce turbulence, aswell as aid in the implantation of the device 1700 within the heart,also as discussed above.

FIG. 18 schematically illustrates an integrally formed ASIC and holderof the invention in a greatly enlarged scale implanted in a wall of theheart after surgery. The implantable sensor device of this embodimentmay include a substantially rigid substrate 1802 including an ASIC 1804,which is located at one end of the substrate 1802. The substrate may beelongated such that a holder portion 1810 is integrally formed therewithand anchored within a wall of the heart such that the ASIC is at leastpartially exposed within the chamber of the heart to be sensed. At aminimum, the active pressure sensors must be exposed. The other end ofthe substrate 1802 is the holder portion that is affixed within a heartwall 36, such that part of the holder portion 1810 of the substrate 1802is located on the other side of the heart wall 36. A wire 1808 may alsobe affixed to the substrate 1802 and connect the ASIC 1804 to an antenna(not shown). The holder portion 1810 of the substrate 1802 may beconfigured to form part of an anchor structure. For example, the outerportion may include a bend, elbow, or other configuration to facilitateanchoring with or without separate affixing means known in the art, suchas sutures or the tobacco pouch suture 1806 schematically shown in FIG.18 surrounding the exposed end of the holder portion 1810.

FIG. 19 schematically illustrates on a greatly enlarged scale anotherintegral ASIC and holder of the invention having a “T” shape anchorimplanted in a wall of the heart. In this embodiment, the substantiallyrigid substrate 1902 includes an ASIC 1908 and an integral holder 1910having an anchor 1906 such that the substrate 1902 and the anchorportion 1906 are configured in a generally “T” shape, with the free endextending through the heart wall. An antenna 1912 may be attacheddirectly to the anchor 1906 and/or substrate 1902. The antenna 1912 maybe in the form of a coil wrapped around the T-shaped end of the holder1910 and connected to the ASIC 1904 by a wire 1908 located on thesubstrate 1902. This may reduce the distance of the wire 1908 requiredor essentially eliminate it.

FIG. 20 schematically illustrates an ASIC and a holder of the inventionin the form of an elbow connector for holding the ASIC and guiding aslack portion of the connecting wire. As illustrated in FIG. 20, anelbow or bend, separately connected to or integrally formed with thesubstrate 2002, may be used to connect and/or guide the wire 2006 to anASIC 2004 held at the other end of the elbow connector. The wire 2006may be connected in such a way as to minimize the movement of the wire2006 at the connection point 2008. As shown, the ASIC 2004 is attachedto the elbow 2002. The wire 2006 is attached to both the ASIC 2004 (at2008) and the elbow 2002 (at 2010). The connection of the wire to theelbow 2002 at 2010 may absorb the stress of wire movement, therebyensuring the fail-safe connection between the wire 2006 and thesubstantially rigid sensor chip 2004 through the numerous cyclesanticipated during the life of a patient. While various embodiments ofthe invention show a holder with an anchor, separate anchor structuremay not be necessary, as the heart wall naturally closes around thesensor device.

The wire 2006 connects the ASIC 2004 to the antenna (not shown) and maybe made of flexible material that allows a signal to be transmittedbetween the sensor structure and the antenna structure. The wire may bemade of gold, platinum, iridium, stainless steel, spring steel, orsimilar material. The wire is totally embedded (surrounded) by aflexible mantle.

As illustrated in FIG. 20, it may be desirable for wire connection tohave some predetermined amount of slack in the wire to account for themovement of the heart without placing undue stress on the components.Thus, the slack may take the form of a bend or loop in the wire 2006 ator near the connection 2010 of the wire 2006 to the bend 2002, as shown.This slack will avoid excessive bending and/or stress on the wire 2006.Slack may also reduce or eliminate additional strain on the heart, asthere is little or no increased effort necessary for normal heartpumping due to the sensor implant. Other configurations to provideslack, reduce bending or stress in the wire 1006 and reduce strain onthe heart may also be used.

FIG. 21 is a three-dimensional representation of a holder of theinvention that mounts the ASIC in the wall of the heart and includessuture wings that limit movement of the ASIC, e.g., during implantationby a doctor, and serve as suture mounts. The implantable sensor device2100 includes a ASIC 2102 having capacitive pressure membranes (notshown) formed on a substantially rigid substrate 2108 having a cut out2110 substantially opposite the pressure membranes as shown anddescribed above. A connector 2114 having electrical wires 2116 and afilament core 2118 is also provided. The device 2100 also includes aholder 2112 (shown in dashed lines) for supporting the ASIC 2102 and thesubstrate 2108 and mounting it to the wall of a heart. A receivingportion 2120 of the holder surrounds and fixedly receives a portion ofthe substrate 2108 such that cut out 2110 is exposed. When the device2100 is inserted into the heart wall, transversely extending flange 2122forms a pair of suture wings that act as stops to ensure that the device2100 is not inserted too far. In addition, the suture wings 2122 may beused in conjunction with sutures for anchoring the device 2100 to theheart. While two wings are shown in FIG. 21, the flange 2122 could beformed with three or more wings if desired. The holder 2112 may alsoinclude a connector attachment portion 2124 to provide a conduit forconnecting the wires 2116 and core 2118 to the sensor chip 2102 via thesubstrate 2108.

FIG. 22 is a three-dimensional representation of another embodiment of aholder of the invention which is similar to the FIG. 21 embodiment butincludes an end cap 2224 to provide further protection to the sensorchip 2202 and substrate 2208, such as when the device 2200 is insertedthrough a heart wall.

FIG. 23 is a three-dimensional representation of a holder of theinvention illustrating how it connects the ASIC via a flexible cable toan antenna. The implantable device 2300 generally includes a holder 2306receiving an ASIC 2302 formed on a substantially rigid substrate 2304. Aflexible connector 2308 electrically connects the ASIC 2302 and anantenna 2310. The entire device 2300, including the holder 2306,connector 2308, and antenna 2310, is encapsulated in a biocompatiblesheathing, such as a seamless sheathing of silicone.

The antenna 2310 serves both as power transducer and antenna, and may befabricated of any type of conductive metal. The antenna 2310 may be madeof pure gold, or any other suitable material, to provide bothbiocompatibility and the necessary degree of electrical conductivity.According to a preferred embodiment of the invention, the antenna 2310and the connector 2308 may be made of the same material, and theconnector 2308 may be part of the antenna 2310, e.g., the connector 2308and the antenna 2310 are integrally formed. The antenna 2310 may beextremely thin (for example, about 25 microns) and light weight. Allcomponents of the implantable device 2300, including the antenna 2310,the connector 2308, and the holder 2306, may be very small and lightweight to avoid strain and irritation of the heart when implanted. Thus,by of example, the holder 2306 may be made of a light weight plastic,and coils in antenna 2304 and wires in connector may be made of arelatively thin and lightweight wire material, such as thin gold orother suitable materials.

The number and size of the coils of the antenna 2310 may be dimensionedin such a way that an appropriate telemetric range between the internaland external coils is achieved. For the embodiment shown in FIG. 23 inwhich the antenna is fixed subcutaneously, the minimum range for thetransmission of measurement data from the internal coil to the extracorporal emitter/receiver unit is about 2 cm to about 25 cm. If theantenna is fixed at or near the heart, the range may be closer to thehigh end, i.e. about 25 cm. However, this required range may changebased on the position of the antenna.

As described above, all energy which is required for the acquisition ofmeasurement data may be provided telemetrically by the antenna 2310,where the coil may be designed as a passive coil. Examples of suitablecoils are illustrated in German patents DE 199 45 879 A1 or DE 101 56469 A1. By way of example, the antenna 2310 may be formed from a cablethat is wound into a plurality of coils. The cable may also connect theantenna 2310 to the sensor chip.

FIG. 24 schematically illustrates an implantable sensor device of theinvention encased in a biocompatible sheathing. A sensor device 2400includes an ASIC 2404 positioned on a substantially rigid substrate 2402and encapsulated in a biocompatible sheathing 2406. The sensor system,i.e., the ASIC 2404, substrate 2402, cable (not shown) and antenna (notshown) may be encapsulated in a biocompatible sheathing such as,silicone, polyurethane or other suitable material. The encapsulation ofthe system preferably is seamless, i.e., has no break or seam. Thisreduces or eliminates the risk of contamination or damage to the sensorsystem structure by fluids within the body. By way of example, thethickness of the encapsulation may be in the range of about 0.01 mm toabout 0.8 mm. A seamless sheathing may be obtained by seamless moldingor by dipping the entire sensor device 2400 (sensor ASIC, cable andantenna) into the biocompatible material.

The implanted device may be positioned in heart to minimize turbulenceof the blood flow within the heart chamber and reactionary forces. Asillustrated in FIG. 25, the sensor device 2400 may be orientated suchthat its shortest side, such as side 2412, may be positioned to be inthe most upstream position in the blood flow path 2416. This presentsthe minimum area in the blood flow and reduces and/or minimizes currentsand reactionary forces caused by the implanted device 2400. Thispositioning may be done regardless of location of the pressure sensors2408 on the chip. As illustrated, the longer sides 2410 of the sensordevice 2400 containing the top surfaces of the capacitive pressuremembranes 2408 may be parallel to the blood flow 2416.

As illustrated in FIG. 25, the shape of the sheathing surface 2414 alsomay be curved or shaped, e.g., similar to the football shape shown inFIG. 25, to further reduce the turbulence caused by blood flow 2416around the sensor device. The biocompatible sheathing 2408 may beapplied to the implanted device 2400 to form the curved surface 2414.Such curves or other shapes may be designed to minimize hydrodynamicforces.

The encapsulation in a fully biocompatible material, such as siliconemay result in very little change in the sensitivity of the pressuresensor. Further, a small offset due to the influence of theencapsulation material may be compensated for during calibration. Thismay allow, for example, measurements of about +/−2 mm Hg or less.

FIG. 26 is a side view of a dual substrate sensor embodiment of theinvention illustrating how the shape of the sheathing may be configuredto minimize turbulence and reactionary fluid forces in the heart. Theimplantable device 2600 includes a substantially rigid ASIC 2602 havingcapacitive pressure membranes 2604 and a substrate 2608 with an aperture2610 substantially opposite the pressure membranes 2604. As illustrated,the device 2600 is encapsulated in a sheathing 2606. The sheathing 2606may be made of a biocompatible material, such as silicone, that isflexible so that the pressure from the blood may be transmitted to thepressure membranes 2604. A pressure transmitting material (not shown)may be placed in the aperture 2610 to aid in transmitting the pressure.The sheathing 2606 may be shaped, such as an oval or footballconfiguration, to reduce or eliminate hydrodynamic forces from the bloodflow 2612. Again, the smallest sides of the device are orientated intothe upstream portion of 2112 of the blood flow.

FIG. 27 is a side view of another embodiment of the inventionillustrating how the shape of the sheathing may be configured tominimize turbulence and reactionary fluid forces in the heart. Thedevice 2700 has the same components as the FIG. 26 embodiment and islike-numbered. However, in this embodiment, the sheathing 2706 may beshaped, such as in a rounded triangle configuration, to reduce oreliminate hydrodynamic forces from the blood flow 2712.

FIG. 28 is a block diagram of the major electronic components of anexternal reader 2800 of the invention for telemetrically receiving datafrom an implanted sensor device. An antenna 2802 may be integrated withthe external reader 2800, as shown schematically in FIGS. 1-2. Also, asillustrated in FIG. 1, the external reader 12 may be attached to thepatient, or alternatively, as illustrated in FIG. 2, the external reader26 may be incorporated inside a handheld device. In other cases, theantenna 2802 may be attached to the patient's body or connected to theexternal reader 2800 by cable connections. Another method attaches theexternal reader to the patient's bed or seat. Numerous such arrangementsmay be employed to adapt to the particular application, as the skilledartisan will recognize.

The antenna 2802 is used for receiving data, in the form of digitalsignals, from the implanted sensor device. The digital signals arereceived at the RF receiver 2806 via an RF generator 2804. The RFgenerator 2804 generates an RF signal to be transmitted via the antenna2802 to power the implanted device.

The digital data signals received by RF receiver 2806 are processed byfilter 2808 and demodulator 2810 before being received and thenprocessed as appropriate by controller 2812. A bidirectional powermodule 2828, described in detail below, is connected between the filter2806 and demodulator 2810. Separate memory devices, such as calibrationmemory 2822, data memory 2824, and parameter memory 2826, may beprovided and communicate with controller 2812. The calibration memory2822 stores calibration information associated with a particular ASICsensor system, and the calibration memory 2822 may store calibrationinformation for a number of different ASIC sensor systems. Calibrationinformation may be obtained from an external source, such as a computer,through communications port 2816. The appropriate calibrationinformation, based on a unique identification number of the ASIC sensorsystem being interrogated, is obtained from the calibration memory 2822.Thus, a medical professional, such as a doctor or nurse, can use onereader to obtain pressure readings from multiple patients.

The data memory 2824 stores data related to the pressure and/ortemperature received from the ASIC of the implanted sensor device. Thedata may be stored in the data memory 2824, and then transferred, via adata memory module 2814, to another device through data communicationsport 2816, such as a computer. Using information obtained from anatmospheric pressure module 2820, the controller 2812 uses the datareceived from antenna 2802 and stored in data memory 2824 to determinethe pressure within the heart, as is known in the art. Using theinformation from the ambient temperature module 2818, the controller2812 also uses the data to determine the temperature within the heart,as is known in the art.

The pressure and temperature calculations, which are performed incontroller 2812, as well as the data from the implanted sensor device,may be stored in the data memory module 2814. These calculations anddata may then be communicated to another device, such as a computerthrough communications port 2816.

The pressure sensor readings, and the parameter alerts described below,may be displayed by the reader on a display (not shown), such as an LCDdisplay or the like. The measured pressure values and parameter alertsalso may be displayed on a monitor of the external reader (not shown)and recorded in an appropriate storage device. The system may beequipped for purposes of telemedicine, so that data is transmitted fromthe external reader to a medical department or healthcare provider viawire connection, telephone, internet or any other suitabletelecommunication source is possible, via known wired or wirelessprotocols.

The parameter memory 2826 stores parameter thresholds. The data receivedfrom the implanted sensor device is compared by the controller 2812 tothe parameter thresholds. If the data fails to meet a particularthreshold, or exceeds a particular threshold, an alarm may occur toalert a user. The threshold parameters may be set by a doctor or otherhealth care professional. By way of example, the threshold range forpressure may set to 25 to 30 mm Hg, depending on the patient. If thereader receives a measurement of 25 mm Hg, which is above the thresholdrange, it alerts the user that the measurement exceeds the threshold.Further, an alert may occur based on the raising of pressure per time.Other parameters may also be used.

The parameter thresholds may be provided to the external reader 2800,such as by a user manually entering a parameter threshold.Alternatively, the parameter thresholds may be provided to the externalreader 2800 from another device, such as computer through communicationsport 2816. The parameter thresholds may be provided via a directconnection, such as by a wire, or by a wireless connection, such as by aLAN, a WAN or the like.

The calibration memory 2822, the data memory 2824 and the parametermemory 2826 may be separate memory storage devices within the externalreader, or each may be a portion of a single memory storage device. Thereader may use a signal at 13.56 MHz, or other known frequencies. Oneexample of a suitable reader is disclosed in published patentapplication No. PCT/EP2004/012670.

Bi-directional power evaluation module 2828 assists in evaluating thestrength of signals received from the implanted device to ensure that aminimum signal strength is received. The signal received from theimplanted devices via antenna 2802 is evaluated by the bi-directionalpower evaluation module 2828. The evaluation may be implemented viavarious methodologies. According to an embodiment of the invention, thereader may increase the power of the signal sent via the antenna 2802 tothe implanted device over small increasing increments. At eachincrement, the bi-directional power evaluation module 2828 evaluates thesignal received back from the implanted device to determine the qualityand strength of the signal. This process is repeated until a successfulsignal is received from the implanted device. The reader than uses theminimum power necessary to achieve an acceptable signal and beginsperforming the reading of data, such as pressure and temperaturemeasurements, from the device. This may be performed by taking apredetermined number of readings (e.g., five readings) in a row. All thereadings may be taken after the minimum power level has been determined.Alternatively, the bi-directional power evaluation module 2828 maydetermine the minimum power level after each of the predeterminedreadings.

By way of another embodiment of the invention, the reader can increasethe power level supplied by the antenna 2802 by larger increments, suchas by quarter Fourier steps (FS steps), until a valid signal is receivedfrom the implanted device. Once a valid signal is obtained, the power isdecreased by one step, such as ¼ FS, then increased in smaller steps,such as ⅛ FS steps) until a valid signal is received. This process isrepeated using progressively smaller steps ( 1/16 FS, 1/32 FS) until aminimum power level is determined. The reader then uses the resultingminimum power level to compute the required power setting and obtains apredetermined number of readings.

Another methodology involves assessing the demodulation quality level(DQL) of the signal in addition to the signal state analysis describedabove. The DQL of a signal changes as the coil geometry and/or distancefrom the reader changes. It does not use an incremental algorithm toassess the required starting power, but the last power setting of thelast measurement. The reader sets the power to a previously used level.If a reading is possible, the reader increases or decreases the powerfor the next reading according to DQL. If no reading is possible, thereader increases the power in increments until valid signal is receivedfrom the implanted device. After a predetermined number (e.g., five) ofsuccessful readings, the reader obtains the measurement readings. Duringthese measurement readings, the reader continues to increase, decrease,or hold the power level according to DQL and obtaining valid signals.

Use of power conditioning may result is various beneficialcharacteristics and features for the voltage controller/stabilizersupply voltage (V_(DDA)) used in the ASIC. When using powerconditioning, there is generally a high common mode rejection ratio(CMRR) for V_(DDA), as well as good radio frequency (RF) suppression forV_(DDA). In addition, a fast power on reset (POR) signal is used ifV_(DDA) falls below tolerance, which would happen if supply power is notsufficient. Because there is no measurement or signal transmission ifPOR not “1,” determining a proper power supply using the powerconditioning may prevent this drawback.

The implantable sensor device of the invention also may be incorporatedor attached to other devices implanted within the body. Examples of suchdevices may include a pacemaker, defibrillator or a drug dispenser.

While the invention has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications in the spirit and scope of theappended claims. For example, while the embodiments described above havebeen directed to implantation of the telemetric sensing device of theinvention within the heart, one or more such devices may be implantedwithin other positions in the cardiovascular system of a patient, suchas the aorta, pulmonary artery, or other great vessel. These examplesgiven above are merely illustrative and are not meant to be anexhaustive list of all possible designs, embodiments, applications ormodifications of the invention.

1. An implantable pressure monitor comprising: a substantially rigidchip including: a proximal end and a distal end; pressure sensorsexposed on a first surface of the chip in a sensor region of the distalend; signal processing circuitry receiving pressure-indicative signalsfrom the sensors and producing pressure-indicative output signals; and achip electrical connector in the proximal end communicating the outputsignals; a substantially rigid substrate that: is spaced apart from thechip; faces the first surface of the chip; is connected to the chipelectrical connector in the chip's proximal end by a substrateelectrical connector; defines an aperture positioned over the sensorregion of the chip's distal end, thereby exposing the pressure sensors;and covers the distal end of the chip except for the sensor region; aflexible filler material located throughout space between the chip andthe substrate except beneath the aperture, thereby leaving the pressuresensors exposed, such that (a) the flexible filler material connects thechip to the substrate, and (b) the distal end of the chip is connectedto the substrate by only the flexible filler material; a wire that:extends from the substrate; is electrically connected to the substrateelectrical connector; communicates the output signals; and is notconnected to the chip; a biocompatible sheath that encapsulates thechip, substrate, filler, and wire, and is sufficiently flexible totransmit pressure exerted on the sheath exterior through the sheath; anda pressure-transferring medium extending from the sheath, through theaperture, and to the pressure sensors, thereby transferring pressureexerted on the sheath exterior to the pressure sensors.
 2. Theimplantable pressure monitor of claim 1, wherein the sheath has a curvedshape to reduce or eliminate hydrodynamic forces.
 3. The implantablepressure monitor of claim 1, wherein the substantially rigid substrateextends proximally from the chip to a proximal end which comprises ananchor.
 4. The implantable pressure monitor of claim 3, wherein theanchor comprises a transversely extending flange that forms suturewings.
 5. The implantable pressure monitor of claim 1, furthercomprising a holder that fixedly receives the chip and the substantiallyrigid substrate and extends proximally to a proximal end which comprisesan anchor.
 6. The implantable pressure monitor of claim 5, wherein theholder further comprises a distal end cap protecting the chip and thesubstantially rigid substrate.
 7. The implantable pressure monitor ofclaim 1, wherein the flexible filler material comprises silicone.
 8. Theimplantable pressure monitor of claim 1, wherein the flexible fillermaterial holds the chip and the substrate together in a fixedrelationship.
 9. The implantable pressure monitor of claim 1, whereinthe substantially rigid substrate extends distally from the chip to adistal end which comprises a barrier wall protecting a distal end of thechip.
 10. The implantable pressure monitor of claim 9, wherein thebarrier wall forms an end cap at the distal end of the chip.
 11. Theimplantable pressure monitor of claim 10, wherein the barrier wallextends in a direction substantially perpendicular to a plane of thesubstrate and to a height such that a top of the barrier wall is at orabove a top of the chip.
 12. The implantable pressure monitor of claim9, wherein the substantially rigid substrate further extends distallyfrom the barrier wall to a tapered front portion.
 13. The implantablepressure monitor of claim 1, wherein the sheath comprises a one-piece,seamless silicone covering.
 14. The implantable pressure monitor ofclaim 13, wherein the flexible filler material comprises silicone. 15.The implantable pressure monitor of claim 1, wherein the substrate issufficiently rigid such that it cannot be folded or rolled up.
 16. Theimplantable pressure monitor of claim 1, wherein the substrate issufficiently rigid to protect the pressure sensors from damage as aconsequence of contact with a surgical instrument during implantationand from mechanical damage during use.
 17. The implantable pressuremonitor of claim 1, wherein the substrate is sufficiently rigid to avoidtwisting of the chip due to turbulent blood flow.
 18. The implantablepressure monitor of claim 1, wherein the substrate is rigid.
 19. Theimplantable pressure monitor of claim 1, wherein the substrate ismechanically inflexible.
 20. A method of monitoring pressure with animplantable pressure monitor positioned in a space in which pressure isto be monitored such that pressure sensors of the monitor are exposed tothe pressure, the method comprising: conveying power to the monitor,thereby causing the monitor to operate; receiving from the monitor thepressure-indicative output signals; and storing the pressure-indicativeoutput signals.